Ensure that input into a MultipleFileProperty properly survies a "round
trip", i.e. what you put in is what you get out, and that using the output
again, as input into another MultipleFileProperty gives the same results.

Proper functionality in a large number of use cases is assured by vastly
improved coverage of unit tests, which was lacking previously.

Simplify parts of the MultipleFileProperty class, including the filename
generation functors I introduced that weren't doing anybody any favours.